1. Field of the Invention
This invention relates to medical imaging systems using nuclear magnetic resonance. In a primary application the invention relates to providing nmr images at lower cost with improved signal-to-noise ratio.
2. Description of Prior Art
Nuclear magnetic resonance or nmr imaging has recently become commercially available and is being enthusiastically received because of its absence of ionizing radiation and its remarkable ability to visualize subtle disease processes. Its high cost, however, tends to limit its use to major medical centers.
A general description of the theory and hardware used in nmr imaging can be obtained from the book entitled Nuclear Magnetic Resonance Imaging in Medicine, published in 1981 by Igaku-Shoin, Ltd., Tokyo.
NMR instruments require a very strong main magnetic field, in addition to a variety of smaller gradient fields. The early nmr instruments used resistive electromagnets to produce a main field in order of 0.5 to 1.5 kilogauss. Although these provided interesting images, the signal-to-noise ratio or snr was inadequate for some studies. The limitation on the magnetic field strength is the power dissipation of the resistive magnet. To overcome this problem, superconductive magnets were introduced to supply the main field. Although these provide the required high fields without dissipation problems they are very expensive, critical to operate and consume relatively large amounts of the liquid helium refrigerant. They do, however, provide strong fields of the order of 5.0 kilogauss. Since the snr varies approximately as the square of the magnetic field, these higher fields are very desirable with all nmr imaging modes.
In addition to imaging it is often desired to provide a spectrum representing the material constituents of a small volume in order to study, for example, phosphorus metabolism. These studies, however, require a very strong main field, usually above 10 kilogauss. At these field strengths, however, imaging is very difficult, if not impossible, because of the tissue attenuation. Therefore a single machine, efficiently providing imaging and tissue analysis, cannot operate at a fixed field strength.
The isolation of the single voxel for tissue analysis can be accomplished in a number of ways. One approach is described by E. R. Andrew in a paper entitled, "Nuclear Magnetic Resonance Imaging: The Multiple Sensitive Point Method," IEEE Transactions on Nuclear Science, Vol. NS-27, June 1980, pp. 1232-1238. In this system time varying gradients of different frequencies are applied to three orthogonal axes. Therefore all regions, except a small voxel, will be time varying and average out to zero. Therefore the resultant signal is due solely to the voxel of interest. Long integration times must be used to separate the fine lines in the spectrum which represent the metabolism-indicating parameters.
One useful approach is to use the imaging system to identify regions of disease, and then use spectral analysis in small regions to evaluate the nature of the disease. However, this requires both imaging and spectroscopic capability, which requires a magnetic field capable of being changed.